1. Field of the Invention
The present invention relates generally to the stirring of sample material and, more particularly, to magnetic stirring arrangements for sample containers of photometric analyzers.
2. Description of the Prior Art
Optical assay procedures are being used increasingly in the qualitative and quantitative analysis of clinical or biological fluids such as blood, urine and the like. Typically, a substance to be assayed and one or more reagents are combined and mixed within a light transparent container. The container, in turn, is positioned in a light path of a photometer for assaying the container contents.
The assay of fluids such as blood or serum in the foregoing manner presents a number of practical difficulties which must be overcome to make such approach commercially attractive from a cost, convenience, and reliability standpoint. In this regard only minute quantities of biological fluid may be available for analysis since it is often impossible or unsafe to withdraw more than a few milliliters of fluid from a patient. The difficulty in assaying small volumes is compounded by the typically viscous nature and the often contaminated state of the sample. Thorough mixing of such samples and the various reagents is essential to derive accurate diagnostic information from the assay reaction. Because reagents for biological assays are usually quite expensive, it is essential that the photometric apparatus be of a nature which minimizes the required quantities of sample and reagents without compromising the reliability or accuracy of the assay.
In U.S. Pat. No. 3,784,170 the foregoing assays are performed in a radiation permeable cell positioned in the path of an analytical light beam and containing a magnetic stirring element for stirring the cell contents. The cell is defined by a pair of parallel side walls closed at each end by a pair of opposing end walls to define a sample chamber of generally rectangular cross section. The magnetic stirring element is positioned in the chamber with its magnetic axis parallel to the side walls, and a rotatable drive magnet outside of the cell rotates the stirring element about an axis perpendicular to the side walls so that the plane of rotation of the stirring element is parallel to the side walls. The optical path of the light beam through the cell is specifically disposed above the path of rotation of the magnetic stirring element so that the stirring element cannot obstruct the optical path and interfere with transmission of light through the cell.
A similar arrangement is disclosed in U.S. Pat. No 3,997,272 except that the magnetic stirring element, generally cylindrically configured, is operatively disposed horizontally in the bottom of the cell. This stirring element is only rotated about its own cylindrical axis and hence remains in the same relative horizontal position at all times.
While the approaches in the two aforementioned patents can be employed with some degree of success, they exhibit a number of drawbacks reducing their overall attractiveness. In the former case, displacing the optical axis vertically above and out of the path of rotation of the stirring element increases the required level of solution in the container and hence requires larger volumes of sample and reagent for each assay. In the latter case the operative horizontal orientation of the stirring element would allow the height of the optical path to be lowered thus enabling smaller sample and reagent volumes to be used. However, rotation of the stirring element about its own horizontal longitudinal axis, as opposed to end over end rotation in a vertical plane, provides relatively inefficient and nonuniform stirring of the sample.